1. Field of the Invention
The present invention relates to a disk memory device for reading data that is recorded on a disk memory medium such as a magnetic disk, an optical disk, or the like and, more particularly, to a data prereading method for prereading data of the disk memory device.
2. Description of the Related Art
In order to increase the speed of reading continuously-arranged data on a disk memory device, there has been employed the following data prereading method. Reading of a data block which follows a data block for which a reading request has been made is started, and the read data are stored in a cache memory before a next reading request is received. When reading requests for continuous data blocks are received; the data which have been preread and stored in the cache memory are transferred, thereby performing a data transfer without being influenced by the disk rotation wait time or the reading head seek time.
As an example of such a prereading method, Japanese Published Patent Application No. Hei. 9-120617 discloses “METHOD FOR REDUCING POWER CONSUMPTION OF DISK DRIVE IN COMPUTER TO REALIZE HIGH SPEED DATA TRANSFER, AND DISK DRIVE CONNECTED WITH COMPUTER”.
In the conventional prereading method, however, the direction along which the data blocks are successively arranged is assumed to be a single direction (the direction along which the logical block address increases) and, therefore, prereading of data cannot be carried out in response to an access for reading data continuously in the backward direction (the direction along which the logical block address decreases).
In recent years, a disk memory device has been increasingly used for recording/playback of video data, audio data, and the like, and the conventional method can provide effective prereading with respect to normal playback. However, when performing a trick play, e.g., a reverse playback, although the data which have already been read out and stored in the cache memory can be played backward, the data which are not stored in the cache memory must be read out successively. Further, during the reverse playback, there occurs a contradiction in that the data that are successive in the forward direction, which are not necessary, are preread, and as a result, an improvement of data transfer by prereading cannot be achieved.
Further, during the trick play such as fast-forward playback or fast-reverse playback, data that are sampled according to the playback speed are read out. In this case, when the conventional prereading method is employed, unnecessary data other than the required data are also stored in the cache memory and, therefore, the cache memory cannot be used effectively. Further, the interval of the required data areas is broadened as the playback speed is increased, which results in a data area that is to be requested is not present in the same track or the same cylinder. In this case, if reading of unnecessary data is performed, prereading of required data cannot be made in time, thereby resulting in an interruption of the playback of video or audio.
Further, in the conventional prereading method, when the preread data are set in the cache memory, the continuity of the preread data blocks that are stored in the cache memory is broken in a reverse playback since the data are stored in the direction along which the cache memory address increase in the order of the preread data. Therefore, in order to secure the continuity of the preread data blocks, it is necessary to form a cache entry, which is entry information into the cache memory, for each preread data block, and to enter the cache entry into the cache list, which results in a detriment to an efficient use of the cache memory.
Furthermore, in the playback that is based on the shuttle dial operation which enables a forward or reverse playback at a playback speed which is suited to a stop angle of a rotation dial that has been mounted on a remote controller of a video tape recorder in recent years, the playback speed can be changed in stages, and the present playback speed is returned frequently to the previous playback speed. In this case, the data that are required in the playback at the previous playback speed cannot be preread by the method of prereading only the data that are required at the present playback speed.
Further, this problem occurs not only in the playback that is based on the above-mentioned shuttle dial operation but also in the playback that is based on the jog dial operation which enables a forward or reverse playback while changing the playback speed from frame-by-frame playback to fast playback according to the speed and direction for rotating the rotation dial.
Furthermore, when a playback start position is specified or when a still picture at a desired position in a played video is output, the playback direction may be frequently switched between the forward playback and the reverse playback by using the above-described shuttle dial operation or the jog dial operation. In this case, since the cache memory does not hold the data that are outputted from the cache memory, the data must be reread from the disk memory medium just after the switching of the playback direction, and thus, an improvement of data transfer by prereading cannot be expected.
The present invention is made to solve the above-described problems. An object of the present invention is therefore to provide a disk memory device which is able to improve data transfer by data prereading, even when a trick play, such as a reverse playback or fast a playback, is carried out.